1. Field of the Invention
This invention relates to a sheet handling unit used in sorting out copy sheets discharged out of a main body of an image forming apparatus such as a copying machine and a printer.
2. Description of the Background Art
Conventionally, there have been known various types of sheet handling units (or post processing units) used with an image forming apparatus such as a copying machine and a printer in which a sorting process, a mechanical sheet process such as stapling and punching, and a stacking process are applied in a series to copy sheets after an image formation by the image forming apparatus.
More specifically, such sheet handling unit is operated such that:
1) in a sorting process, copy sheets discharged out of a main body of an image forming apparatus are sorted out on bin trays that are movable up and down in synchronism with a sheet discharge operation to obtain sheet sets respectively placed on the bin trays; PA1 2) in a mechanical sheet process, a stapling or a punching operation is applied to each of the sheet sets on the bin trays; and PA1 3) in a stacking process, the sheet sets having been applied with the mechanical sheet process thereto are discharged onto a common sheet stacker via a specified transport route in a state that the sheet sets are stacked one over another.
The sheet handling unit having the above arrangement has suffered from various drawbacks. One of the drawbacks is non-aligned stacked state of sheet sets on the sheet stacker due to the following two main reasons.
One reason is that a next sheet set S2 coming onto the sheet stacker displaces the previously stacked sheet set S1 forward due to a frictional force generated between these sheet sets when the next sheet set S2 is sliding over the previous sheet set S1. The result is a non-aligned stacked state of sheet sets on the sheet stacker.
Another reason is that in the case where a stapling is applied to the lead end of the sheet set with respect to the sheet set discharge direction, the lead end of the next sheet set S2 is tripped over the staple(s) applied to the previous sheet set S1, resulting in displacing (pushing) forward the previous sheet set S1. The result is also a non-aligned stacked state of sheet sets on the sheet stacker.
Further, the above sheet handling unit has confronted another problem, damage of the sheet surface of sheet sets in contact with each other due to a staple applied to the sheet set S2 or to the sheet set S1.